The substantial removal of some or all of a particulate material from a fluid stream, e.g. gas or aqueous stream, can be important for many reasons including safety and health, machine operation, and aesthetics. Filter media materials are used in filtration structures placed in the fluid path to obtain physical separation of the particulate from the fluid flow. Filter media are desirably mechanically stable, have good fluid permeability, relatively small pore size, low pressure drop and resistance to the effects of the fluid such that they can effectively remove the particulate from the fluid over a period of time without serious mechanical media failure. Filter media can be made from a number of materials in woven, non-woven or film material forms. Such materials can be air laid, wet laid, melt blown, or otherwise formed into a sheet-like material with an effective pore size, porosity, solidity, or other filtration requirements.
Material and non-woven filter elements can be used as surface loading media. In general, such elements comprise porous films or dense mats of cellulose, cellulose derivatives, glass, PTFE, synthetic polymers, and fibers oriented across a stream carrying particulate material. The media is generally constructed to be permeable to the fluid flow, and to also have a sufficiently fine pore size and appropriate porosity to inhibit the passage of particles greater than a selected size therethrough. As materials pass through the media, the upstream side of the media operates through diffusion and interception to capture and retain selected sized particles from the fluid (gas or liquid) stream. The particles are collected as a dust cake on the upstream side of the filter media, in the case of a gas stream, for instance. In time, the dust cake also begins to operate as a filter, increasing efficiency. This is sometimes referred to as “seasoning,” i.e. development of an efficiency greater than initial efficiency. PTFE materials and similar microporous materials, primarily operate as surface loading or barrier filters.
Dense woven and nonwoven fabrics can operate as a combination of surface loading media and depth media, wherein the particles are trapped throughout the depth of the media. The pore size of the fabrics is dependent upon the size and density of the fibers and the process by which they are formed. The efficiency of the filter media is dependent upon many parameters including the depth of the filter media, pore size, and electrostatic nature of the material. However, it is often desirable to fine-tune the pore properties of depth media as exemplified in the following patents and patent applications.
Carlson, et al., in U.S. Pat. No. 4,629,652, discloses a process for providing a palletized aerogel comprising a support structure to a silicon-based pre-gel heated to supercritical conditions. Upon venting the fluid phase under supercritical conditions, the aerogel forms on and/or within the support structure. This method of solvent removal avoids the inherent shrinkage of the solid product that occurs when conventional drying techniques are employed. Martin, in U.S. Pat. No. 5,156,895, discloses a body including a support structure in which is formed monolithic aerogel. One aspect of the method of making the body includes a solvent substitution step and a supercritical drying step. In both of these cases, the aerogel is a covalently bonded cross-linked network.
Woven and nonwoven fabrics are also used extensively in the protective apparel and building products markets. A key characteristic of barrier products is the ability to allow passage of air, while inhibiting the passage of particles, water, and other liquids. WO 2004/027140 entitled “Extremely High Liquid Barrier Fabrics,” for instance, discloses many aspects of barrier fabrics.
In US 2004/0213918, Mikhael, et al., discloses a coating process that allows modification of the surface properties of a porous substrate without changing significantly the air permeability. This process is described as being accomplished by controlling the coating of individual fibers in ultra-thin layers that do not extend across the pores in the material.
In humid environments, for instance, air conditioning units, and water filtration systems, biofouling of porous substrates can be a significant issue in determining the service life of filtration media and barrier fabrics. The initial interactions between a bacterial cell and a solid surface are determined by physico-chemical events, primarily ionic and hydrophobic interactions. The initial events result in a reversible adherence of the cell to the surface; as time progresses this adherence becomes irreversible due to the synthesis of exopolymeric materials which eventually encase the cell in a matrix comprising the bulk of the biofilm. In general, bacterial cell surfaces tend to be anionic and hydrophilic, thus negatively charged and/or hydrophobic surfaces tend to be more difficult to colonize. Disruption of the initial events by alteration of the surface charge or moderate increases in hydrophobicity has been shown to lead to a reduction in the ability of the bacteria to colonize the surface.
At the same time, methods for preventing biofouling are desired that do not include the use of highly toxic compounds. Needed are processes to provide porous materials with resistance to biofouling while maintaining filtration and/or barrier properties and without using toxic materials.